Photosensitive elements which can be used in image-reproduction processes are well-known in the graphics arts industry. Such elements are exposed to actinic radiation, usually through an image-bearing transparency such as a color separation transparency, to produce an image which is either positive or negative with respect to the transparency used. Positive-working elements produce a duplicate image with respect to the transparency used for exposure. Negative-working elements produce an image that is the reverse of the transparency through which they are exposed.
After imagewise exposure, the photosensitive elements may be developed in a variety of ways, e.g., by washout of soluble image areas, application of colorant material which adheres to the tacky image areas, peeling apart of photoadherent layers, or combinations of these techniques. The resulting image-bearing photosensitive elements are particularly useful for color proofing applications. A useful reference for color proofing methods is Principles of Color Proofing, by Michael H. Bruno, GAMA Communications, Salem, NH, 1986.
Cohen and Fan, U.S. Pat. No. 4,174,216, describe negative-working photosensitive elements which can be used to produce colored images by a dry process in which a colorant material is applied to develop the image. The element comprises, in order from top to bottom, a strippable coversheet, a photoadherent layer, an essentially nonphotosensitive tonable contiguous layer, and a support. After imagewise exposure to actinic radiation through the coversheet, the element can be developed by peeling off the coversheet and applying colorant material. The areas of the photoadherent layer which have been exposed to actinic radiation adhere to and are removed with the coversheet thus uncovering the underlying tonable contiguous layer in those areas. The unexposed areas of the photoadherent layer remain on the supported tonable contiguous layer. When the colorant material is applied it adheres selectively to the uncovered areas of the tonable contiguous layer, corresponding to the exposed areas, and does not adhere to the remaining areas of the photoadherent layer. Thus a colored image which is the reverse of the transparency used for exposure is formed. By repeating this process several times, each time using a new element and a different color separation transparency, and toning with the corresponding colorant, it is possible to build up a multicolored surprint proof.
While the above process yields image reproductions with high resolution in a fast and economical way, the reproductions sometimes exhibit defects known as "fine toned line defects". These are random defects that appear as very thin lines of color where there should be no color. A fine toned line defect can make the image unusable, especially if it occurs in a critical area, e.g., a face or a clear background area. An entire multilayer, multicolor proof can be ruined if the final toned layer has fine toned line defects in a critical area. As thinner photopolymer layers are used in order to improve resolution, fine toned line defects become even more numerous.
Fine toned line defects are thought to be caused by microscratches on the coversheet of the photosensitive element. The scratches, probably caused by abrasion of the coversheet and/or the photosensitive film layer during the manufacturing process, are only about a micron (.mu.m) wide. However, during the peel-apart step, these micron-wide scratches cause strips of unexposed photopolymer about 20 microns in width to be peeled off with the coversheet. During the subsequent toning step, the areas where strips of unexposed photopolymer have been removed will accept toner and thus appear as colored lines.
Attempts to eliminate fine toned line defects by eliminating all scratches on the coversheet films have been largely unsuccessful. It is extremely difficult to eliminate all sources of dust and other contaminants which may scratch the surface of the coversheet film in any one of several processing steps. Therefore, other avenues must be explored to achieve a multilayer peel-apart photosensitive element which does not exhibit fine toned line defects.